An analytical model is developed for natural convection from a single circuit board in a sealed electronic equipment enclosure. The circuit card is modeled as a vertical isothermal plate located at the center of an isothermal, cuboid shaped enclosure. A composite model is developed based on asymptotic solutions for three limiting cases: pure conduction, laminar boundary layer convection, and transition flow convection. The conduction shape factor and natural convection models are validated using data from CFD simulations for a wide range of enclosure geometries and flow conditions. The model is shown to be in good agreement, to within 10% RMS, with the numerical data for all test configurations.